This invention relates to the separation of mixtures of colloidal aluminosilicate particles to extract from them on the one hand a high purity imogolite like fibrous aluminosilicate polymer and on the other hand allophane like spherical aluminosilicate particles.
Imogolite is an aluminosilicate polymer that occurs in the form of fibers that are most probably hollow. Imogolite occurs naturally in volcanic ash and in certain soils. Natural imogolite is impure, being mixed with other aluminosilicates such as allophanes and/or raft-like boehmite. Natural imogolite cannot be used in this impure form in currently known applications, such as for use as a basic constituent for antistatic layers designed for example for photographic products as described in European Patent 0 714 668. In the same way, allophanes, if mixed wither other aluminosilicates, cannot be used in their impure form in their own applications, such as for use as gelling agent for cosmetics.
There exist various processes for synthesizing imogolite of ranging purity. For example U.S. Pat. Nos. 4,152,404 and 4,252,779 of Farmer describe a process for preparing an inorganic material analogous to natural imogolite. European Patent 0 741 668 describes a process to obtain imogolite of high purity with an Al:Si molar ratio of between 1 and 3. The imogolite obtained is rid of charges, such as salts, by dialysis. However, this process will not separate allophanes, which are uncharged, from the imogolite.
According to the known literature, for example Journal of Soil Science, 1979, 30, 347, S. I. Wada, A. Eto, K. Wada, a pure imogolite has an Al:Si molar ratio close to 2. It is specified here that the terms xe2x80x9cvery high purityxe2x80x9d or xe2x80x9chigh purityxe2x80x9d designate an aqueous solution containing at least 80% and preferably at least 90% imogolite by weight. The term xe2x80x9cimogolite likexe2x80x9d means a composition of anisotropic, fibrous aluminosilicate particles with an Al: Si molar ratio of between 1 and 3, and preferably close to 2. These particles are at least 1 xcexcm long and have a diameter of about 20 nm.
The process described in European Patent 0 741 668 is relatively long and has to be followed scrupulously if high purity imogolite is to be obtained. In particular it is absolutely necessary, during the digestion and filament growth step, to keep the pH and Al+Si concentration within precisely defined ranges of values. If the synthesis is uncontrolled, then the formation of silica gels, boehmite or allophanes is observed. Boehmite has a non-fibrous structure with an Al:Si molar ratio greater than 4. Allophanes are spherically shaped aluminosilicate particles that are smaller than imogolite particles and have a different Al:Si ratio. The term xe2x80x9callophane likexe2x80x9d means a composition of hollow spherical aluminosilicate particles, with an Al:Si molar ratio of between 1 and 4. These particles have a diameter of between 3,5 and 5,5 nm. In addition allophanes are amorphous (P. Bayliss, Can. Mineral. Mag., 1987, 327) whereas imogolites are crystalline.
If the synthesis has not been sufficiently well controlled, then the imogolite obtained is not pure enough for known applications and so is useless. As the same way allophanes, being mixed with the imogolite, are also useless in their own applications.
This invention provides a method for obtaining, simply and cheaply, on the one hand very high purity imogolite and on the other hand allophanes from a mixture of colloidal aluminosilicate particles, i.e., an impure imogolite solution deriving from either a natural source or an uncontrolled synthesis.
The present invention is a method for separating a mixture of colloidal aluminosilicate particles in aqueous dispersion, comprising the step of passing said mixture through an ultrafiltration unit whereby a permeate and a retentate are obtained, and an imogolite like fibrous aluminosilicate polymer with an Al:Si molar ratio between 1.8 and 2.5 is obtained in the retentate, and allophanes like spherical aluminosilicate particles are obtained in the permeate.
Preferably, the imogolite like fibrous aluminosilicate polymer obtained in the retentate has an Al:Si molar ratio between 1.9 and 2.
Imogolite obtained in the retentate and allophanes obtained in the permeate are recovered and can then be used in their own applications.